The present invention generally relates to ultrasonic probes and more specifically to ultrasonic probes for acoustic imaging.
Ultrasonic probes provide a convenient and accurate way of gathering information about various structures of interest within a body being analyzed. In general, the various structures of interest have acoustic impedances that are different than an acoustic impedance of a medium of the body surrounding the structures. In operation, such ultrasonic probes generate a broadband signal of acoustic waves that is then acoustically coupled from the probe into the medium of the body so that the acoustic signal is transmitted into the body. As the acoustic signal propagates through the body, part of the signal is reflected by the various structures within the body and then received by the ultrasonic probe. By analyzing a relative temporal delay and intensity of the reflected acoustic waves received by the probe, a spaced relation of the various structures within the body and qualities related to the acoustic impedance of the structures can be extrapolated from the reflected signal.
For example, medical ultrasonic probes provide a convenient and accurate way for a physician to collect imaging data of various anatomical parts, such as heart tissue or fetal tissue structures within a body of a patient. In general, the heart or fetal tissues of interest have acoustic impedances that are different than an acoustic impedance of a fluid medium of the body surrounding the tissue structures. In operation, such a medical probe generates a broadband signal of acoustic waves that is then acoustically coupled from a front portion of the probe into the medium of the patient's body, so that the signal is transmitted into the patient's body. Typically, this acoustic coupling is achieved by pressing the front portion of the probe into contact with a surface of the abdomen of the patient. Alternatively, more invasive means are used, such as inserting the front portion of the probe into the patient's body through a catheter.
As the acoustic signal propagates through the patient's body, portions of the signal are weakly reflected by the various tissue structures within the body and received by the front portion of the ultrasonic medical probe. As the weakly reflected acoustic waves propagate through the probe, they are electrically sensed by electrodes coupled thereto. By analyzing a relative temporal delay and intensity of the weakly reflected waves received by the medical probe, imaging system components that are electrically coupled to the electrodes extrapolate an image from the weakly reflected waves to illustrate spaced relation of the various tissue structures within the patient's body and qualities related to the acoustic impedance of the tissue structures. The physician views the extrapolated image on a display device coupled to the imaging system.
Since the acoustic signal is only weakly reflected by the tissue structures of interest, it is important to try to provide efficient acoustic coupling between the front portion of probe and the medium of the patient's body. Such efficient acoustic coupling would insure that strength of the acoustic signal generated by the probe is not excessively diminished as the signal is transmitted from the from portion of the probe into the medium of the body. Additionally, such efficient acoustic coupling would insure that strength of the weakly reflected signal is not excessively diminished as the reflected signal is received by the front portion of the probe from the medium of the body. Furthermore, such efficient acoustic coupling would enhance operational performance of the probe by reducing undesired reverberation of reflected acoustic signals within the probe.
An impediment to efficient acoustic coupling is an acoustic impedance mis-match between an acoustic impedance of piezoelectric materials of the probe and an acoustic impedance of the medium under examination by the probe. For example, one piezoelectric material typically used in ultrasonic probes is lead zirconate titanate, which has an acoustic impedance of approximately 33.times.10.sup.6 kilograms/meter.sup.2 second, kg/m.sup.2 s. The acoustic impedance of lead zirconate titanate is poorly matched with an acoustic impedance of human tissue, which has a value of approximately 1.5.times.10.sup.6 kg/m.sup.2 s.
Previously known acoustic coupling improvement schemes have had only limited success and have created additional manufacturing, reliability and performance difficulties. For example, one previously known scheme provides an ultrasonic probe of high-polymer piezoelectric elements. Each of the high-polymer piezoelectric elements comprises a composite block of piezoelectric and polymer materials. For example, FIG. 1 is a cross sectional view of a typical piezoelectric composite transducer. As shown, a single piezoelectric ceramic plate is reticulately cut to be finely divided, so that a number of fine pole-like piezoelectric ceramics 1 are arranged two-dimensionally. A resin 7 including microballoons (hollow members) 6 is cast to fill in gaps between piezoelectric ceramic poles 1. The resin is cured so as to hold the piezoelectric ceramic poles 1. Electrodes 4, are provided on both end surfaces of the piezoelectric ceramic poles 1 and the resin 7, so as to form the piezoelectric ceramic transducer. The piezoelectric composite transducer shown in FIG. 1 is similar to one discussed in U.S. Pat. No. 5,142,187 entitled "Piezoelectric Composite Transducer For Use in Ultrasonic Probe" and issued to Saito et al. Because this patent provides helpful background information concerning piezoelectric composites, it is incorporated herein by reference.
While composite materials provide some improved acoustic coupling to various desired media, there are difficulties in electrically sensing reflected acoustic waves received by such composites. A dielectric constant of each high polymer element is relatively small. For example, for a composite that is 50% polymer and 50% piezoelectric ceramic, the dielectric constant measurable between electrodes of the high polymer element is approximately half of that which is inherent to the piezoelectric ceramic. Accordingly, the dielectric constant measurable between the electrodes of the high polymer element is only approximately 1700. A much higher dielectric constant is desirable so that a higher capacitive charging is sensed by the electrodes in response to the reflected acoustic waves. The higher dielectric constant would also provide an improved electrical impedance match between the probe and components of the imaging system electrically coupled to the probe.
Another previously known acoustic coupling improvement scheme provides an ultrasonic probe comprising one or more layers of dissimilar matching materials bonded to a front portions of a piezoelectric vibrator body. A thin layer of a cement adhesive is applied to bond each layer, thereby creating undesirable adhesive bond lines between the layers of dissimilar materials and the piezoelectric body.
For example, FIG. 2 illustrates an ultrasonic transducer 202 comprising an acoustically damping support body 204 of epoxy resin having an acoustic impedance of 3.times.10.sup.6 kilograms/meter.sup.2 second, kg/m.sup.2 s, a piezoelectric vibrator body 206 of a piezoceramic, such as lead zirconate titanate having the acoustic impedance of 33.times.10.sup.6 kg/m.sup.2 s, a silicon layer 208 having an acoustic impedance of 19.5.times.10.sup.6 kg/m.sup.2 s, and a polyvinylidene fluoride layer 210 having an acoustic impedance of 4 * 106 kg/m.sup.2 s. The silicon and polyvinylidene fluoride layers are used to match the relatively high acoustic impedance of the piezoceramic material of the vibrator body to a relatively low acoustic impedance of human tissue, which has an acoustic impedance of 1.5.times.10.sup.6 kg/m.sup.2 s. The vibrator body 206 shown in FIG. 2 has a resonant frequency of 20 megahertz, MHz, and the silicon and polyvinylidene fluoride layers each have a respective thickness that is a quarter wave length of the resonant frequency of the vibrator body.
Electrodes (not shown in FIG. 2) are electrically coupled to the vibrator body 206 for electrically sensing acoustic signals received by the transducer. Unlike the piezoelectric composite discussed previously herein, the piezoceramic material of the vibrator body 206 has a relatively high dielectric constant, which is not degraded by polymer. For example, lead zirconate titanate has a relatively high intrinsic dielectric constant of approximately 3400.
The piezoelectric vibrator body 206 shown in FIG. 2 is connected on one side to the acoustically damping support body 204 by means of an adhesive layer over a large area, and is attached on an opposite side at least indirectly to the silicon layer 208 by another adhesive layer. Similarly, the polyvinylidene fluoride layer 210 is connected to silicon layer by yet another adhesive layer. The thickness of each adhesive layer is typically 2 microns. The ultrasonic transducer 202 shown in FIG. 2 is similar to one discussed in U.S. Pat. No. 4,672,591 entitled "Ultrasonic Transducer" and issued to Briesmesser et al. Because this patent provides helpful background information concerning dissimilar matching materials bonded to piezoelectric bodies, it is incorporated herein by reference. Though the dissimilar layers employed in previously known schemes help to provide impedance matching, the adhesive bonding of these layers creates numerous other problems. A plurality of bonding process steps needed to implement such schemes creates manufacturing difficulties. For example, during manufacturing it is difficult to insure that no voids or air pockets are introduced to the adhesive layer to impair operation of the probe. Furthermore, reliability of this previously known transducers is adversely effected by differing thermal expansion coefficients of the layers of dissimilar materials and the piezoelectric block. Over time, for example over 5 years of use, some of the adhesive bonds may lose integrity, resulting in "dead" transducer elements that do not effectively transmit or receive the acoustic signals. Additionally, operational performance is limited at higher acoustic signal frequencies, such as frequencies above 20 megahertz, by the bond lines between the piezoelectric body and the dissimilar materials.
One measure of such operational performance limitations is protracted ring down time in impulse response of the ultrasonic transducer of FIG. 2. Such impulse response can be simulated using a digital computer and the KLM model as discussed in "Acoustic Waves" by G. S. Kino on pages 41-45, which is incorporated herein by reference. FIG. 3 is a diagram of the simulated impulse response of the ultrasonic transducer of FIG. 1 having the resonant frequency of 20 Megahertz, radiating into water, and constructed in accordance with the principles taught by Briesmesser et al. In accordance with the impulse response diagram shown in FIG. 3, simulation predicts a -6 decibel, db, ring down time of 88.637 nanoseconds, nsec, a -20 db ring down time of 270.411 nsec, and a -40 db ring down time of 452.350 nsec.
What is needed is a reliable ultrasonic probe that provides enhanced operational performance and efficient electrical coupling to imaging system components, while further providing efficient acoustic coupling to the desired medium under examination by the probe.